The invention relates to synthetically commutated variable displacement pumps in which individual cylinders are electronically commutated and their displacement is selected by a controller. Such pumps are for example sold under the names Digital Displacement® Pump or DDP® pump. The electronic valves (14,18,24) on each cylinder (4) can be controlled such that, for every cycle the piston (6) reciprocates, the volume of fluid pumped can be controlled (so called synthetic commutation/synthetically commutated pump/and further below a synthetically commutated pump/motor). Each of these is an example of an electronically controlled displacement selecting machine. Each cylinder (4) can be viewed as an independent pump and is controlled independently. By combining the output of the cylinders in groups, a service is created. If all cylinders in the pump are combined a single service is created. For example, a synthetically commutated pump with 12 cylinders all combined to a single service will have very high dynamic range of flow output and very low output pressure pulsation. This same pump can have the output of the cylinders arranged such that it has 4 services with 3 cylinders each, or in any other combination. These cylinders may or may not all be on the same eccentric but would ideally be equally spaced in terms of the mechanical angle between the cylinders. These 4 services are completely independent even though they are packaged within the same pump. Defining the number of services is a matter of creating the galleries to interconnect the desired cylinders and defining the interconnections in the control software. This creation of services is not as obvious as with typical swash plate, vane, and gear pumps as there may not be any obvious separation between the services. For example a service can be made up of cylinders on different eccentrics or even from a pump in another housing. An eccentric may be driving cylinders where each cylinder provides flow to a different service. The standard definition of a physical pump and output becomes blurred very quickly.
The synthetically commutated pump can also be configured to not only pump but also to motor the pressure energy from the hydraulic output back into the crankshaft (8). This is referred to as a Digital Displacement® Pump/motor or DDPM™ or a synthetically commutated pump/motor.
The concept of hydraulic stiffness: “
A “hard” hydraulic fluid (high bulk module) transmits pressures very fast and leads to a stiff hydraulic system. This is appreciated in closed loop controlled systems. “Stiff” systems are achieved by small pressurized volumes, hard surrounding walls (pipes instead of flexible hoses) and high viscose fluids. Beside that pressure increases the bulk module of mineral oil. A “soft” hydraulic system is more subject to instability, but it is in general quieter, because high frequent pressure ripple is damped better
“(Catalogue HY14-3200/US, Parker Hannifin Corporation)
Hydraulic compliance in this context refers to the compressibility of the working fluid, but primarily to the containment of the fluid; e.g. hoses (any part of containment with flexible walls) will expand when fluid pressure increases, allowing an increase in volume of liquid stored. Similarly, hydraulic accumulators provide intentionally large amounts of compliance. A simple open tank exhibits compliance, since an increase in volume of contained liquid results in a pressure increase due to gravity. Bubbles within the working fluid will also provide compliance. Compliance in a fluid drive system changes the dynamic response when trying to control the system. The term ‘hydraulic compliance’ is not limited to the bulk modulus of the working fluid and the walls containing and constraining that volume, and is meant to encompass the effect of stiffness of components directly influencing the characteristics and compressibility of the hydraulic volume. However, the bulk modulus alone may represent a significant compliance. For example, in another fluid system the fluid alone may compress by about 1.5% volume at 2,000 psi, about 3% volume at 5,000 psi, and about 6% volume at 10,000 psi. Another metric used for compliance is the amount of fluid that is swept by a fluid machine piston stroke in relation to the system volume. If the relative swept volume is small, then many strokes are needed to raise the system pressure from low pressure to high or working pressure. If the relative swept volume is large, then few strokes are needed to raise the system pressure from low to high or working pressure. It is considered that a system requiring 5 or fewer fluid machine piston strokes to raise system pressure to working or high pressure is a hydraulically stiff system. Therefore by definition a system requiring 6 or more fluid machine piston strokes to raise system pressure to working or high pressure, is a hydraulically soft one. The compliance of the system is influenced by the compressibility of the working fluid which may be affected by the fluid type, fluid temperature, fluid air content/aeration, age of the fluid, and other factors obvious to one skilled in the art. The compliance of the system is similarly influenced by the constraint of the system holding the fluid which may be affected by similar factors, for example the ambient (atmospheric) pressure. The consumer state influences system compliance, and a prime example is the extension of a ram which increases the system volume and increases the exposed area of the containment/constraining walls, thus increasing the compliance.
Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.
When a fluid working machine operates as a pump, at least one low pressure manifold (16, 26) typically acts as a net source of fluid and a high pressure manifold (20) typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold (20) typically acts as a net source of fluid and at least one low pressure manifold (16,26) typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” are relative, with the relative pressures being determined by the application. In some embodiments of the present invention the pressure within the at least one low pressure manifold (16,26) is significantly higher than atmospheric pressure, for example, several atmospheres, however, it will be less than the pressure in the high pressure manifold (20) during normal operation. A fluid working machine may have more than one low pressure manifold (16, 26) and more than one high pressure manifold (20).
In systems with a single pump and multiple actuators there is always undesirable compromise given the practical impossibility of matching the instantaneous pressure requirements of all of the active actuators to the single pressure supply.
In the case of the state-of-the-art “load sensing” system, the displacement of a variable displacement pump is controlled such as to maintain its output pressure to a fixed margin above the maximum pressure required of any of the loads. The difference between this pressure and the actual pressure required of any one of the loads is throttled in a proportional valve, creating energy losses. When only one actuator is moved at a time these systems can be reasonably efficient. However when multiple actuators must be moved simultaneously at different pressures then the efficiency becomes poor—depending on the duty cycle, these losses can cause the overall efficiency of such a system to reduce to 30%.
The pump/motor described in EP 0494236 B1 (Artemis Intelligent Power Ltd) and sold under the trade mark Digital Displacement® is a positive-displacement fluid pump/motor in which the working volumes are commutated not by mechanical means but by electronically-controlled solenoid-actuated poppet valves (so called synthetic commutation of a pump/motor, or synthetically commutated pump/motor). Control of flow is achieved by varying the time-averaged proportion of working volumes which are commutated such as to pump fluid from the low pressure port to the high pressure port (“pump enabled”), or which are commutated such as to motor fluid from the high pressure port to the low pressure port (“motor enabled”), to the proportion which are connected in both expansion and contraction strokes to the low pressure port and thus do no fluid work (“idled”). A controller, synchronised to the position of the shaft by means of a position sensor, supplies pulses to the solenoid coils at the appropriate times such as to commutate each working volume as desired. Because the commutation of each stroke of the working volume is independently controllable, the pump/motor is capable of supplying fluid to or absorbing fluid from a port, in individual discrete volume units, each corresponding to a single stroke or part of a stroke (see WO 2004/025122) of a single working volume. The high pressure port of each working volume may be connected to a different fluid circuit. Thus a single pump/motor composed of many working volumes may provide multiple independent fluid supplies or sinks, the flow to or from each of which is independently variable.
By way of background art, U.S. Pat. No. 7,543,449 B2 (Cnh America Llc, Ivantysynova et al.) introduces, and discloses the term ‘displacement control’, and includes explanatory passage: “ . . . displacement-controlled systems are used in which an adjustable pump that is variable in its displacement volume is used for the control or regulation of the motion of the hydraulic motor(s). The consumer is hence controlled only via the volume flow provided by the pump, without the use of a control valve or similar device in the main circuit.” So, a variable displacement pump is used to control the motion of a hydraulic motor. There is no additional control valve/similar.